The invention relates to an improved solar energy collecting device of the type wherein a heat transport liquid is periodically circulated between a solar energy collection area and a thermal energy storage and exchange device. In particular, the invention constitutes a specific improvement in the construction of the double walled thermal energy absorbing unit employed in the solar energy collection apparatus described and illustrated in detail in U.S. Pat. No. 4,232,655 to Frissora et al.
In the Frissora et al. patent, a solar energy collection apparatus is disclosed comprising a plurality of double walled energy absorbing glass tubes. Such tubes have the space between the walls evacuated and the inner wall has a solar energy absorbing coating on its exterior surface. One end of the inner tube is provided with connection means for the successive introduction and withdrawal of a heat transfer fluid, generally water, from the interior of the inner tube. To conveniently permit the periodic filling of the inner tube with the heat transfer fluid, and the subsequent withdrawal of the heat transfer fluid, a venting tube is provided which extends into the upper portions of the inner wall tube and provides at its lower end a connection to a venting manifold which normally communicates with the top of a fluid storage tank. Such venting tube permits the ready exhaustion of air from the inner tube during the initial filling operation and then functions to control the maximum height of the heat transfer fluid introduced into the inner tube by draining off such fluid through the vent tube at a rate equal to that at which it is introduced from the supply manifold. Care must be taken, however, to insure that the rate of supply of heat transfer fluid does not exceed the rate of withdrawal of fluid through the venting tube so as to effect the forcible discharge of all air from the top of the inner heat transfer tube. Once a solid column of water is established in such tube, undesirable siphoning action can take place which would have a deleterious effect on the operation of the entire system.
Those skilled in the art will recognize that any constricted area vent tube can become accidentially clogged with particulate or fibrous material accidentally introduced into the system. If this occurs, it becomes impossible to fill the tube to the desired level, and, more importantly, to withdraw all of the heat transfer fluid due to creation of a vacuum block at the top of the fluid in the tube.